float getValue( int slice, int pixel,
	vector<float> &drs_basemean,
	vector<float> &drs_gainmean,
	vector<float> &drs_triggeroffsetmean,
	UInt_t RegionOfInterest,
	vector<int16_t> AllPixelDataVector,
	vector<int16_t> StartCellVector
){
	const float dconv = 2000/4096.0;

	float vraw, vcal;

	unsigned int pixel_pt;
	unsigned int slice_pt;
	unsigned int cal_pt;
	unsigned int drs_cal_offset;

	// printf("pixel = %d, slice = %d\n", slice, pixel);

	pixel_pt = pixel * RegionOfInterest;
	slice_pt = pixel_pt + slice;
	drs_cal_offset = ( slice + StartCellVector[ pixel ] )%RegionOfInterest;
	cal_pt    = pixel_pt + drs_cal_offset;

	vraw = AllPixelDataVector[ slice_pt ] * dconv;
	vcal = ( vraw - drs_basemean[ cal_pt ] - drs_triggeroffsetmean[ slice_pt ] ) / drs_gainmean[ cal_pt ]*1907.35;

	return( vcal );
}



// FACT raw data is stored in fits files in large vector<int16_t>. these
// containt the data of the full camera for a single event.
// in order to work with this data, one needs to extract the part, which
// belongs to the current pixel and apply a DRS calibration, based on a dedicated
// DRS calibration file.
//
// TODO: The calibration Data as well as the Data and StartCell vectors are a bit
// massive to give this function as parameters.
// better to have these vectors somehow bundled in classes like:
// RawData - class and
// DrsCalibration - class
// those classes would be the result of
// openDataFits() and
// openCalibFits()
//
size_t applyDrsCalibration( vector<float> &destination,
	int pixel, 
  int LeaveOutLeft, 
  int LeaveOutRight,
	vector<float> &drs_basemean,
	vector<float> &drs_gainmean,
	vector<float> &drs_triggeroffsetmean,
	UInt_t RegionOfInterest,
	vector<int16_t> AllPixelDataVector,
	vector<int16_t> StartCellVector,
	int verbosityLevel
){
	// In order to minimize mem free and alloc actions
	// the destination vector is only resized, if it is way too big
	// or too small
	size_t DestinationLength = RegionOfInterest-LeaveOutRight-LeaveOutLeft;
	if ( destination.size() < DestinationLength || destination.size() > 2 * DestinationLength ){
		destination.resize( DestinationLength );
	}

	//	We do not entirely know how the calibration constants, which are saved in a filename.drs.fits file 
	// were calculated, so it is not fully clear how they should be applied to the raw data for calibration.
	// apparently the calibration constants were transformed to the unit mV, which means we have to do the same to
	// the raw data prior to apply the calibration
	//
	// on the FAD board, there is a 12bit ADC, with a 2.0V range, so the factor between ADC units and mV is
	// ADC2mV = 2000/4096. = 0.48828125 (numerically exact)
	//
	// from the schematic of the FAD we learned, that the voltage at the ADC 
	// should be 1907.35 mV when the calibration DAC is set to 50000. 
	// 
	// One would further assume that the calibration constants are calculated like this:

	// The DRS Offset of each bin in each channel is the mean value in this very bin,
	// obtained from so called DRS pedestal data
	// Its value is about -1820 ADC units or -910mV

	// In order to obtain the DRS Gain of each bin of each channel
	// again data is takes, with the calibration DAC set to 50000
	// This is called DRS calibration data.
	// We assume the DRS Offset is already subtracted from the DRS calibration data
	// so the typical value is assumed to be ~3600 ADC units oder ~1800mV 
	// As mentioned before, the value *should* be 1907.35 mV 
	// So one might assume that the Gain is a number, which actually converts ~3600 ADC units into 1907.35mV for each bin of each channel.
	// So that the calibration procedure looks like this
	// TrueValue = (RawValue - Offset) * Gain
	// The numerical value of Gain would differ slightly from the theoretical value of 2000/4096.
	// But this is apparently not the case. 
	// The Gain, as it is stored in the DRS calibration file of FACT++ has numerical values 
	// around +1800.
	// So it seems one should calibrate like this:
	// TrueValue = (RawValue - Offset) / Gain * 1907.35

	// When these calibrations are done, one ends up with a quite nice calibrated voltage.
	// But it turns out that, if one returns the first measurement, and calculates the mean voltages 
	// in each bin of the now *logical* DRS pipeline, the mean voltage is not zero, but slightly varies 
	// So one can store these systematical deviations from zero in the logical pipeline as well, and subtract them.
	// The remaining question is, when to subtract them.
	// I assume, in the process of measuring this third calibration constant, the first two 
	// calibrations are already applied to the raw data. 

	// So the calculation of the calibrated volatage from some raw voltage works like this:
	// assume the raw voltage is the s'th sample in channel c. While the Trigger made the DRS stopp in its t'th cell.
	// note, that the DRS pipeline is always 1024 bins long. This is constant of the DRS4 chip. 

	// TrueValue[c][s] = ( RawValue[c][s] - Offset[c][ (c+t)%1024 ] ) / Gain[c][ (c+t)%1024 ] * 1907.35 - TriggerOffset[c][s] 
	
	const float dconv = 2000/4096.0;
	float vraw;

	if ( RegionOfInterest != AllPixelDataVector.size()/1440 ){
		if (verbosityLevel > 0){
			cout << "RegionOfInterest != AllPixelDataVector.size()/1440" << endl;
			cout << "RegionOfInterest: " << RegionOfInterest << endl;
			cout << "AllPixelDataVector.size(): " << AllPixelDataVector.size() << endl;
			cout << "aborting" << endl;
		}
		return 0;
	}

	// the vector drs_triggeroffsetmean is not 1440 * 1024 entries long
	// but has hopefully the length 1440 * RegionOfInterest (or longer)
	if ( drs_triggeroffsetmean.size() < 1440*RegionOfInterest ){
		if (verbosityLevel > 0){
			cout << "Error: drs_triggeroffsetmean.size() < 1440*RegionOfInterest" << endl;
			cout << "drs_triggeroffsetmean.size():" << drs_triggeroffsetmean.size() << endl;
			cout << "RegionOfInterest" << RegionOfInterest << endl;
			cout << "aborting" << endl;
		}
		return 0;
	}

	int DataPos, OffsetPos, TriggerOffsetPos;
	
	for ( unsigned int sl = LeaveOutLeft; sl < RegionOfInterest-LeaveOutRight ; sl++){

		DataPos = pixel * RegionOfInterest + sl;
		// Offset and Gain vector *should look the same
		OffsetPos = pixel * drs_basemean.size()/1440 + (sl + StartCellVector[pixel])%(drs_basemean.size()/1440);

		TriggerOffsetPos = pixel * drs_triggeroffsetmean.size()/1440 + sl;

		vraw = AllPixelDataVector[ DataPos ] * dconv;
		vraw -= drs_basemean[ OffsetPos ];
		vraw -= drs_triggeroffsetmean[ TriggerOffsetPos ];
		vraw /= drs_gainmean[ OffsetPos ];
		vraw *= 1907.35;

//		slice_pt = pixel_pt + sl;
//		drs_cal_offset = ( sl + StartCellVector[ pixel ] ) % RegionOfInterest;
//		cal_pt    = pixel_pt + drs_cal_offset;
//		vraw = AllPixelDataVector[ slice_pt ] * dconv;
//		vcal = ( vraw - drs_basemean[ cal_pt ] - drs_triggeroffsetmean[ slice_pt ] ) / drs_gainmean[ cal_pt ]*1907.35;
//		destination.push_back(vcal);

		destination[sl-LeaveOutLeft] = vraw;
	}

	return destination.size();
}